A disparity in illegal drug use that disadvantages individuals in the lower socioeconomic strata has significantly increased in the last two decades. New evidence indicates that this increase results in large part from a changing influence of socioeconomic status (SES) on drug use, and not from a negative influence of drug use on SES that has accumulated over time. However, the SES-related influences responsible for this growing disparity remain largely unknown. . The aims of this study are threefold. First, it will address limitations in current evidence on the growing disparity of illegal drug use across socioeconomic strata. Using the most current, available data the project will (a) update analyses of socioeconomic trends in cocaine past 1998 (b) examine socioeconomic trends up to the present in illegal drugs that have not yet been examined, such as marijuana, methamphetamine, ecstasy, and 'blunts,1 (c) examine whether the growing disparity sterns from differential incidence and/or differential duration of drug use across socioeconomic strata, and (d) examine SES indicators other than education. These analyses will use the National Survey on Drug Use and Health, a nationally- representative, cross-sectional survey of drug use administered yearly, although between 1979 and 1988 it was administered only every third year. Second, the study will predict both changes in the prevalence of illegal drugs over time and also changes in the characteristics of illegal drug users over time. We use a "Diffusion of Innovation" framework, which predicts that users of an innovation such as illegal drugs fall into distinct categories such as "innovators," "early adopters," and "late adopters." People in each of these categories are theorized to differ from each other in terms of sociodemographic, psychological, and environmental characteristics. Furthermore, the profiles of drug users at one point in time are expected to predict the prevalence of drug use in the future. The third aim of the project is to trace the extent to which the key predictors of illegal drug use, as highlighted by the Diffusion framework, have their roots in the early life course. Using a longitudinal survey that has followed 838 children from 1st grade to age 30, the analysis will examine how far back in the life course current predictors of illegal drug use extend. We expect that many early predictors have not yet been linked to later drug use because the current profile of illegal drug users, which includes factors such as low socioeconomic status, did not emerge until recently. Results from this project will inform policy and intervention programs aimed at reducing the prevalence of illegal drug use across all socioeconomic strata.